Radial block copolymers are known and it is also known that during their manufacture up to 20 wt % of the diblock copolymers remain unreacted and are present as diblock copolymer material. These low diblock content radial copolymers have been proposed as components in pressure-sensitive adhesives, where they are used to make during label manufacture, a laminate of a face stock, pressure-sensitive adhesive layer, and a release liner, such as silicone-coated paper, which is passed through an apparatus that converts the laminate into commercially useful labels and label stock. The converting operation processes involve printing, die-cutting, and matrix stripping to leave labels on a release liner, marginal hole punching, perforating, fan folding, guillotining and the like. It is important that the cutting action breaks the face stock and adhesive layer, but does not indent the release liner. Producing a series of labels on a backing sheet involves cutting around the label and removing the material between two labels (the matrix) while leaving the label itself attached to the backing sheet. It is important that the die-cutting machine make a clean break at operating speeds. The adhesive with the copolymer of low diblock content is formulated to have the desired viscoelastic and adhesive properties so that it can be applied to the release liner or the face-stock back, and will remain on the label after stripping and will have the required adhesion. But these are properties that make the adhesive film difficult to cut or break. These properties make die-cutting difficult and inconsistent, causing the adhesive lends to form adhesive strings and deposits on the cutting blade. FIG. 1 illustrates a typical die-cutting process.
Die-cutting involves cutting the laminate through to the release liner face. Other procedures involve cutting completely through the label laminate and include hole punching, perforating, and guillotining, particularly on flat sheets.
The cost of converting a laminate into a finished product, such as a label, is a function of the various processing operations' rates. Line speed depends on whether a printing step is involved. If there is no printing as with, for example, computer labels, speeds can reach 300 meters/minute. If label printing is involved, then speeds of 50-100 meters/minute are typical. While the nature of all laminate layers impact convertibility cost, the adhesive layer can limit convertibility ease. The adhesive layer's viscoelastic nature causes this limitation—in particular its high elasticity prevents it from flowing away from the cut line during die-cutting and also promotes its transfer to cutting blades during cutting. High adhesive elasticity also causes adhesive stringiness, which hinders matrix stripping as the unwanted facing material is removed after die-cutting. High elasticity also promotes adhesive layer reconnection after the layer is severed.
Achieving good convertibility does not necessarily coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit needs, and important properties include peel adhesion, tack, shear, and viscosity at various temperatures and adhesion on various substrates such as polymers, papers, glasses, and steels. Good, general-purpose adhesives may exhibit poor convertibility simply because the adhesive is difficult to cleanly sever. The adhesive may stick to a die or blade. As previously discussed in label manufacture, die-cutting and matrix stripping operations occur at speeds from 5-300 meters per minute, typically 50-100 meters per minute, if printing is involved. Within a range of speeds, use of a particular adhesive may result in breaking the matrix despite the fact that successful matrix stripping can occur at speeds on either side of the breaking speed. One goal is to provide adhesive systems where the adhesive has good die-cutting performance and where the matrix can be successfully stripped over the entire operating speed range.
Typical label adhesives are produced from acrylic polymer emulsions, which may be tackified by hydrocarbon or natural-resin tackifiers. While these have good die-cutting performance, they require handling large volumes of liquid and subsequent liquid removal. Accordingly, adhesives applied as hot melts would be preferred. At low temperature, acrylic-based adhesives perform poorer than hot-melt systems. Moreover, hot melts can be used at faster line application speeds in a broader temperature range, have more aggressive tack, and can be used under humid conditions. It is however important that the adhesive has desired theological properties both for processability such as coating and at end use temperature.
Hot-melt pressure-sensitive adhesive systems are well known and consist of tackified thermoplastic elastomers such as styrenic block copolymers together with tackifying resin(s) and generally some plasticizing oil, an antioxidant and optionally fillers. Styrenic block copolymers containing polystyrene and polybutadiene blocks and/or polyisoprene blocks are particularly useful. These materials are generally available as pure triblocks, (sometimes referred to as SIS and SBS copolymers), and diblocks (sometimes referred to as SI and SB copolymers). The materials are also available as mixtures of diblock and triblock materials (sometimes referred to as SIS+SI and SIS+SB). Examples of these materials are the Vector materials marketed by Dexco and the Kraton D materials marketed by Kraton Polymers. Radial block copolymers have also been proposed.
It is known to use diblock/triblock blends as the elastomeric component in hot-melt pressure-sensitive adhesives. It is further known that adhesive properties and viscosity can be controlled by varying the diblock-to-triblock ratio, varying the styrene content, varying the polymer molecular weight, and varying the block molecular weights within the polymers. The melt viscosity can also be controlled by the addition of plasticizing oils and varying the molecular weight of the polymers. Examples of materials that have been used are Kraton D 1113, containing 16% styrene and 56% diblock; Quintac 3433, marketed by Nippon Zeon, containing 55% diblock and 17% styrene; Vector 4114, containing 42% diblock and 17% styrene; and Vector 4113 containing 20% diblock and 17% styrene. Vector 4114 and Vector 4113 are Dexco products. While these materials have good adhesive properties when tackified and can be used in hot melts for label production, they do not have optimum die-cutting properties. Furthermore, their balance of adhesive properties is not optimum.
U.S. Pat. No. 5,663,228 concerns improving label adhesive die-cuttability. But the proposed solution is different and more complicated than the present invention and requires the use of two particular block copolymer resins having certain glass-transition temperatures and the choice of a tackifying resin that, when mixed with the two particular block copolymers, increases the difference between the two block copolymers' glass transition temperatures. U.S. Pat. No. 5,663,228 also does not appreciate the importance of the adhesive's elastomeric behavior under die-cutting conditions. Examples of styrenic copolymers that are used in the adhesive mixtures of U.S. Pat. No. 5,663,228 are Finaprene 1205 available from AtoFina and Kraton 1107 available from Kraton Polymers.
U.S. Pat. No. 5,412,032 concerns linear SIS triblock/diblock copolymers that can be used in labels to improve die-cutting. This is accomplished using block copolymers with a styrene content from 18 to 24 wt %, a polystyrene block molecular weight from 25,000 to 35,000 an overall molecular weight of above 280,000 up to 520,000 and a coupling efficiency of 20% to 40%. The coupling efficiency corresponds to the percentage of triblock material in the overall block copolymer.
PCT Patent applications PCT/US01/20671 and PCT/US01/20609 describe the use of certain diblock/triblock blends and the use of tetrablock and pentablock copolymers in label adhesives to improve die-cutting performance.
It is also known to use radial block copolymers in hot melt adhesives. For example, U.S. Pat. Nos. 5,194,500 and 5,750,607 relate to styrene-isoprene three-arm block copolymers and their use in adhesives. These three-arm radial copolymers are available as Kraton 1124 from Kraton Polymers and Quintac 3450 and Quintac 3460C from Nippon Zeon. International Patent Publications WO 92/20725 and WO 95/14727 are concerned with radial block copolymers comprising polystyrene block segments and diene block segments, the diene block segment is preferably predominately polyisoprene block containing a small amount of butadiene at the end of the diene block to ensure multi arm coupling. These publications also disclose the use of these polymers in hot melt adhesive systems. WO 92/20725 is primarily concerned with the use of such polymers in adhesives used in disposable articles. WO 95/14727 is concerned with achieving optimum balance between high holding power and low melt viscosity of the adhesives.
European Patent Application 0798358 A1 is concerned with hot melt adhesives, particularly hot melt adhesives for labeling which have a reduced viscosity. The adhesives have a low diblock content and we have found that this results in an adhesive that is too cohesive and has high elasticity which is detrimental for die cuttability as is shown in Comparative Example 1 which is based on the radial polymer DPX-551 mentioned in European Patent Application 0798358 A1 as a suitable polymer for use in its adhesive formulations.
The radial block copolymers of WO 95/14727 are characterised by the formula:(pS-pI-pB)nX  (1)wherein pS is polystyrene, pJ is polyisoprene, pB is polybutadiene, X is a residue of a multifunctional coupling agent used in the production of the radial block copolymer, and n is a number greater than or equal to 3 and representative of the number of branches appended to X. According to WO 95/14727 the number n is predominately 4. The molecular weight of the pS block of the radial block copolymer is between about 10,000 to about 15,000 g/mole, preferably from about 12,000 to about 14,000 g/mole. The pJ-pB block preferably has a total average number molecular weight (polystyrene equivalent molecular weight) ranging from about 40,000 to about 130,000 g/mole, preferably from about 50,000 to about 115,000 g/mole. The overall number average molecular weight (polystyrene equivalent) of the radial block copolymer ranges from about 200,000 to about 400,000 g/mole, preferably from about 225,000 to about 360,000 g/mole, and the polystyrene block pS component is present in an amount of at least about 14 to about 24 parts, preferably from about 15 to about 22 parts, per 100 parts by weight of the radial block copolymer.
The radial block copolymers of WO 95/14727 are thus constituted of polystyrene block segments and polydiene block segments in accordance with formula (1). The copolymers may be random, tapered, block or a combination of these, provided that the polybutadiene segment acts as the terminus segment of the polydiene block so that it may react with the coupling agent. The other end block of the polymer is polystyrene.
The pS segment is generally prepared by sequentially polymerizing styrene. In accordance with formula (1), isoprene is employed to make the pJ segments, the (pS-pJ) polymer chains being formed by sequential polymerization of isoprene with the pS. The pS-pJ-pB-Li polymer chains are then formed by the sequential polymerization of living pS-pI-Li polymer chains with butadiene.
The radial or multiblock (pS-pI-pB)nX copolymers are correspondingly made by coupling the pS-pJ-pB-Li living polymer chains with a multi- or tetra-functional coupling agent, such as SiCl4. Thus, the styrene is polymerized to form pS, the isoprene is then introduced to form pS-pI, the butadiene is then introduced to form pS-pI-pB, and the pS-pJ-pB chains are then coupled with the tetrafunctional coupling agent to form the (pS-pI-pB)nX radial or multiblock polymer. The polymer is generally recovered as a solid such as a crumb, powder or pellet.
In the pJ-pB segment of the (pS-pI-pB)nX polymer, the polyisoprene is present in an amount sufficient to impart predominantly polyisoprene characteristics, not butadiene or polybutadiene characteristics, to the polymer. Thus, in the pI-pB segments of the polymer, the weight amount of polyisoprene will exceed 50% of the total weight of diene in the polymer, i.e., pI/(pI+pB)>50 wt %. Conversely, the weight amount of butadiene or polybutadiene will be less than 50% of the total weight of diene in the polymer, i.e., pB/(pI+pB)<50 wt %. Preferably, the polybutadiene portion of the diene segment is less than 10%, most preferably less than 5%, based on the total weight of the (pI+pB), or diene component of the polymer.
The small amount of butadiene at the end of the diene midblock is useful in that it enhances the coupling reaction in formation of the radial polymer, and results in a radial polymer with a higher number of branches.
The radial polymers of WO 95/14727 are thus synthesized by first contacting styrene with an initiator, suitably, for example, a sec-butyllithium initiator, in the presence of an inert diluent, for example, cyclohexane. A living polymer is then formed, as represented, for example, by the simplified structure pS-Li. The living polystyrene polymer pS-Li is next reacted with an isoprene monomer; the resulting product being represented by the simplified structure pS-pI-Li. The living polymer pS-pJ-Li is then reacted with a small amount of butadiene monomer to produce a living polymer with the structure pS-pI-pB-Li, pB represents butadiene or polybutadiene. Coupling of the pS-pJ-pB-Li with the coupling agent produces a branched block copolymer with the structure (pS-pI-pB)nX. The radial polymer that is produced, using SiCl4 as a coupling agent, will render (pS-pI-pB)nX polymer where n is predominantly 4, i.e. more than 50 wt % of the radial copolymer is four-arm. The butadiene need be added only in an amount necessary to assure that the ends of all of the pI segments of the polymer chains are provided with at least one molecule of butadiene, though as suggested the butadiene can be added in larger or smaller amounts.
Coupling agents which may be used to produce the radial polymers of WO95/14727 include those possessing four sites reactive toward carbon-lithium bonds. Suitable coupling agents are those compositions of the formula X(L)n where X represents the coupling moiety residue, and L is suitable leaving group. Exemplary of coupling agents of this type are silicon halides, for example, SiCl4, or a silane compound where one or more of the halides is substituted by an alkoxy group, for example, tetramethoxysilane or tetraethoxysilane compounds, epoxy compounds, for example, epoxidised linseed oil, epoxidised soybean oil; acrylate multi esters, for example, pentaerythritol tetraacrylate; epoxy silanes, divinyl compounds, for example, divinyl benzene, and the like.
In addition to polystyrene, other alkenyl aromatic hydrocarbon monomers, such as alkyl-substituted styrenes, alkoxy-substituted styrenes, 2-vinyl pyridine, 4-vinyl pyridine, vinyl naphthalene, alkyl-substituted vinyl naphthalenes and the like. For simplicity herein, the terms styrene, styrenic, polystyrene content- and polystyrene equivalent molecular weight as used in this application are intended to include these other alkenyl aromatic hydrocarbons.
The isoprene polymerization technique is preferably such that the stereochemistry of the polymerisable monomer is adjusted so that predominantly cis-1,4-polyisoprene having a glass transition temperature of less than −50° C. as measured by differential scanning calorimetry at a 10° C. per minute temperature scan rate is produced.
The radial block copolymers are preferably produced by solution anionic techniques, although they could be prepared using bulk, solution or emulsion techniques. Such techniques entail contacting the monomers to be polymerized simultaneously or sequentially with an organoalkali metal compound in a suitable solvent at a temperature within the range from about −100° C. to about 150° C., preferably at a temperature within the range from about 0° C. to about 100° C. Particularly effective anionic polymerization initiators are organolithium compounds having the general formula:RLin wherein:R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted aromatic hydrocarbon radical having from 1 to about 20 carbon atoms; and n is an integer of 1 to 3.
In general, any of the solvents known to be useful in the preparation of such polymers may be used. Suitable solvents include straight- and branched chain hydrocarbons such as pentane, hexane, heptane, octane and the like, as well as alkyl-substituted derivatives thereof, cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, cycloheptane and the like, as well as alkyl-substituted derivatives thereof, aromatic and alkyl-substituted aromatic hydrocarbons such as benzene, toluene, xylene and the like; hydrogenated aromatic hydrocarbons, such as tetralin, decalin and the like. Linear and cyclic ethers such as dimethyl ether, methyl ethyl ether, anisole, tetrahydrofuran and the like may be used in small amounts.
During the coupling reaction involved in producing radial block copolymers not all the polymer will be coupled. The coupling efficiency of radial block copolymers is defined as the mass of coupled polymer divided by the mass of coupled polymer plus the mass of uncoupled polymer. The coupling efficiency herein refers to that of the original polymer not including any degradation fragments formed during processing. Thus, when producing the (pS-pI-pB)nX branched polymers, the coupling efficiency is shown as a percentage by the following relationship:
            mass      ⁢                          ⁢      of      ⁢                          ⁢      coupled      ⁢                          ⁢      polymer              mass      ⁢                          ⁢      of      ⁢                          ⁢              (                  uncoupled          +          coupled                )            ⁢                          ⁢      polymer        ×  100  ⁢          ⁢      (    %    )  Coupling efficiency can be measured by an analytical method such as gel permeation chromatography.
Coupling efficiency can be controlled by a number of methods. One method to reduce coupling efficiency is to add less than the stoichiometric amount of coupling agent required for complete coupling of the polymers. Another means of reducing coupling efficiency is by the premature addition of a terminator compound. These terminators, such as water or alcohol, react very quickly and can easily be employed to cut short complete coupling of the polymers. In addition, by performing the coupling reaction at elevated temperatures, such as above about 90° C., thermal termination of many of the living polymer groups (pS-pI-Li) occurs prior to coupling. The typical coupling conditions include a temperature of between about 65° C. to about 75° C. and sufficient pressure to maintain the reactants in a liquid phase.
Following the coupling reaction or when the desired coupling efficiency has been obtained, any remaining uncoupled product is terminated such as by the addition of terminators, for example, water, alcohol or other reagents, for the purpose of removing the lithium radical forming the nucleus for the condensed polymer product. The product is then recovered such as by coagulation utilizing hot water or steam or both, or alternatively by the use of a devolatilizing extruder.
Radial four arms block copolymers and their use in hot melt adhesives are also described in European Patent Application 1103577 A1 and U.S. Pat. No. 5,292,819.
The three and four arms products of these patents and the commercially available materials suffer from the disadvantages that they do not have optimum theological properties for use in permanent label adhesives. We have found that they have a coupling efficiency greater than 60%, generally greater than 70% and accordingly contain less than 40 wt % of diblock copolymer. These polymers tend to have too high a tensile strength and are harder and too cohesive to be useful in adhesive formulations and in other applications such as sound deadening, shock absorption and polymer modification.
We have now developed radial block copolymer compositions which overcome these problems.
We have found that, unlike the known products, if the diblock copolymer content of a mixture of a radial styrenic block copolymer and a styrenic diblock copolymer is above 40 wt % of the total block copolymer content, an adhesive system having improved theological and improved die-cutting performance with desirable adhesive properties may be obtained. Some or all of the diblock may be produced during the manufacture of the radial copolymer.
Surprisingly, we found that die-cutting takes place at relatively low deformation rates and involves pushing the adhesive to the side of the line of cut rather than involving a sharp cutting action. In successful die-cutting, the adhesive must creep when subjected to cutting knife action, flow away from the cut point, and not reform over the cut line.
The creep of the adhesive may be illustrated by assuming typical conditions of die-cutting operations, i.e. a machine line speed of 100 m/min, a rotating cylinder of 10 cm diameter, and face paper and adhesive layers with a thickness of 80 and 20 microns, respectively. Since the diameter of the rotating cylinder is much larger (by a factor 100) than the overall thickness to indent, the effective vertical motion is only 10 cm/s when the knife starts to indent the face paper, and only 2 cm/s when the adhesive itself is indented.
The second aspect has been discovered with the help of finite-element simulations of the die-cutting process performed with Abacus Software. These showed that the adhesive is pushed away by the much stiffer face paper, well before the cutting knife starts to indent the adhesive layer. In other words, the adhesive layer flows under the pressure imparted by the cutting knife on the face stock, which covers the adhesive layer. In most instances, no direct contact between the knife and the adhesive layer occurs.
FIG. 2, is an illustration of a die-cut label during the die cutting process, in which 1 is the release paper, 2 the adhesive layer, 3 is the frontal paper and 4 is the die-cutting blade which is moving in an anticlockwise direction to make the cut. The simulation illustration shows how as the knife crushes and breaks through the paper, the adhesive layer is pushed away under the paper from the line of cut, but that the knife itself does not cut through the adhesive layer. Accordingly, the more readily the adhesives flow and the less elastic they are, the easier and cleaner the cut will be.
Altogether, both the surprisingly low deformations rates involved in the die-cutting process, as well as the need for the adhesive layer to undergo permanent flow during die-cutting operations explains why water-based acrylic adhesives behave better than their triblock (for example, SBS or SIS) counterparts. These two systems provide good examples of good and bad die-cutting behavior respectively.
Viscoelastic behavior of hot-melt adhesives at a given temperature is conveniently captured by the two dynamic moduli known as G′ and G″, the loss modulus G″ giving an indication of the viscous behavior, and the storage modulus G′ giving an indication of the elastic behavior. The ratio of G″ and G′ is known as the loss factor Tangent delta (Tan δ).
The finding that the cutting mechanism pushes the adhesive away from the line of cut rather than performing a sharp cut, leads to the conclusion that the adhesive should be less elastic to enable it to permanently flow away from the line of cut at the cutting temperature, normally room temperature. Emphasis should be put on the low frequency behavior because of the surprisingly small values for the vertical velocity of the knife during die-cutting operations.
Dynamic mechanical analysis of acrylics systems shows indeed that the storage modulus G′ continuously decreases with frequency, with no indication of a constant plateau at low frequencies. At the same time, there is a relatively high loss modulus G″ at low frequency, essentially overlaying with G′. This amplifies the tendency of the adhesive to undergo permanent deformation and flow under stress, as shown in FIG. 3. On the other hand, similar analysis of previous pure triblock copolymer based adhesives shows a constant and relatively high plateau modulus G′ (>10,000 Pa) in the low frequency region, much higher than the loss modulus G″, reflecting the tendency for the adhesive to recover from deformation, which is undesirable for die-cutting.
We have found that there is also a marginal difference at high frequency, between the behavior of acrylics and the systems of the present invention (glass transition region and glassy domain), especially in the glass transition location on the frequency axis. The theological behavior at these frequencies can be modified by changing the tackifier package, which is known to minimally influence die-cutting behavior.
Accordingly, we have found that, to have good die-cutting performance, an adhesive based on radial copolymers should fulfill the following criteria:
G′ at room temperature should decrease monotonically with frequency, at frequencies below the glass transition region (typically <10 rad/s), down to a constant storage modulus plateau at the lowest frequencies. The elastic modulus plateau should be lower than 8,000 Pa, preferably lower than 6,000 Pa, more preferably lower than 5,000 Pa, most preferably lower than 4,000 Pa, when measured at 20° C.
G′ should intersect a value of 10,000 Pa at a frequency that is preferably higher than 0.001 rad/s; preferably higher than 0.01 rad/s more preferably higher than 0.05 rad/s; most preferably higher than 0.1 rad/s, when measured at 20° C.
The loss factor Tan δ defined as the ratio G″/G′ preferably comprises between 0.2 and 1.3, more preferably between 0.2 and 1.0, more preferably 0.3 to 1.0, more preferably 0.4 to 1.0, most preferably 0.6 to 1.0, at the frequency at which the storage modulus intersects a value of 10,000 Pa, when measured at 20° C.
We have found that, in addition to the improved adhesion performances, desirable die-cuttability properties may be achieved using an adhesive system containing a styrenic block copolymer which contains a radial block copolymer and at least 40 wt % of a diblock styrenic copolymer.